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ALL DfMFWS(C*S t hg J Heat sink construction details

In the rectifier circuit there are two avenues to follow. You can buy four diodes and make your bridge or you can do as I did and use one of the nifty one-inch-square epoxy bridge rectifiers The little one-inch jobs are convenient because you don't have to mess around figuring which end is the anode and which is the cathode. Ordinarily, the epoxied bridges are simply marked AC, AC, + # and - . Can you beat that?

As always, no matter what you do for rectification, be sure your rectifier is rated for the current you will be needing. Most of the little square bridges are rated between 20 and 35

Amps. I am using a Sern-tech-Alpac 7905 only because t happened to have one on hand. Motorola. International Rectifier, VARO, and EDI make excellent equivalents.

Voltage regulation depends on adequate filtering and an IC known as a 7812 After much experimentation, I found that my voltage regulation {as well as hum attenuation) improved as I increased the value of filter capacitor C2 Starting out with 2000 uF, I worked my way upward to 1 J,000 uF. i hough I now have a 37,000-uF filter capacitor in (he circuit, 13,000 uf seemed to be enough, t he amount of filtering achieved by going from

Parts List

— 13,000-uF. 25-V efectrolytic capacitor C2—IO^uF, 25-V electrolytic capacitor C3—0.22-uF. 100-V tubular capacitor C4, C5—0.01-uFt 500 V ceramic capacitor 01D4—25-A diodes or epoxy bridge rectifier (see text) Dl 05— 1N4004 diodes F1 — Fuse, 5 Amp F2—Fuse, 30 Amp

Rt —120-0hnri 4-W resistor

R3—500-0hmt 1-W potentiometer

JG1 — 7812 voltage regulator

S2—6-position wafer switch

Miscellaneous: NET neon bulb, binding posts, line cord, 0-25rV dc vottmeter, 0-30A ammeter, heat sinks, chassis, bfower, fuseholders;

and bulb socket.

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13,000 to 37,000 uF is very, very slight and detectable only with a scope Obviously one can't ignore the thought that if 13,000 uF is good, a higher value would be better, but let me caution you enthusiastic high-capacity freaks against installing 150,000-uF capacitors without limiting inrush current I haven t experimented beyond 37,000 uF

The 7ti12 voltage regulator is an IC device capable of maintaining excellent regulation as long as the input voltage falls between 14.6 and 19 volts nominally. A number of companies are producing the 7812 and it generail\ has some sort of prefix or suffix, but the digits remain the same.

In this circuit, the 7812 is above ground through a 200- to 500-0hm resistor, f don't put an exact value on this because it is not that critical. Going from receive to full-output transmit on my Atlas (300 mA to peaks of 16 Amperes), the voitage drop on the power supply \s 0,4 V, whit h ain't bad Since I normally don't run my equipment at full bore, the drop from receive to transmit is quite small

\s was the case with the pass ;ransistors, I mounted the 7812 on a heat sink affixed to a small piece of Plexiglas on standoffs (to simplify its isolation from the chassis). The heat sink {see Fig 2) is made of four strips ot one-inch wide aluminum cut at varying lengths and bent up a half-inch at each end I then placed each one "inside ' a larger one until, voila, La Heat Sink a la Cheap. To keep the strips aligned, a hole was drilled which also served to attach the 7812,

While it isn't necessary, you can build in a selectable voltage feature by connecting any number of 1N4004 diodes on a wafer-

type switch, This switch goes between pin 3 of the 7812 and ground. {If this seems like a ¡ot of hooey to you, you may disregard the above and connect pin 3 of the 7812 to ground through R'i. You will see a voltage change of approximately 07 V with each position on the switch With my supply, I have the capability of as much as 15 V or so, and the switch permits me to "switch down to' the proper voltage I desire [13 8 V

1 he value of bleeder resistor R1 across the output is not critical either, but have something there for your protection

By varying the resistance of R3, your output voltage will vary considerably I believe a potentiometer instead of a fixed-value resistor is a better route so that more flexibility is available for future voltage needs which now might not be considered As rn my case, if you are receiving 16 8 V rrom your transformer, 2r>0 Ohms is sufficient to yield the 13,8 V dc you want.

Should you be supplying your rectifier with 16 to 18 volts and not be getting stable 13 volts or soP check to be sure thai you are not losing (dropping) all of your voltage in your rectifier diodes or epoxy bridge Some of the epoxy bridge rectifiers are poor in the area of voltage consistency Try a different one, even ot llit1 same manufac turer, Another place to watch for voltage losses is in your wiring The more current you draw, the higher your volt^ age drops may become in your transformer, rectiiier filter capacitor, or wiring Wire which is too small may cause substantial voltage drops. 1 would suggest using #14 AWG wire at least.

H*u-nvr?vnvm

My first test of the power supply was disastrous. Not

Fig 3. Power supply metering arrangement.

only was the regulation terrible, but the audio was 80% hum, 20% ham. Two things lead to the elimination of hum: First (and already covered), I placed my voltage regulator above ground on the Plexiglas support; second, I connected all of my chassis ground connections to one point

As with my other homebrew endeavors, I first mounted the power supply on an open chassis, ttread-boarding can save you much agony when it comes time to actually fitting the darned thing in a permanent box Scouting around M the Dayton Hamvention, I was able to come up with a perfect cabinet (which formerly was a microvolt meter) for >1.00. When shopping tor an enclosure, don't overlook old, non working test equipment, etc

Metering can be added easily as shown in Fig i. (When will the price of meters ever come down')

The cost is going to vary depending on the state of your junk box and what kind of hamfest bargains you can locate, I spent more than I really wanted to, and that was slightly over ten dollars. You can't beat the pages of 7i Magazine for bargains on the components used here; it was from there that I purchased all of my purchased parts.

As you build this, take your time, Do a good job. Dress all of your leads. Use red wires for + and black wires for - lake time to consider the aesthetics of this project Then tuck it away in a dark corner and ignore Jtr because it's only a stupid power supply ■

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